1. Field of the Invention
The present invention pertains to the field of interfacing electrical components. More particularly, this invention pertains to the field of reducing ringing of a digital signal delivered over a transmission line.
2. Background of the Related Art
In the field of electronics, information is commonly communicated using digital waveforms that are ideally represented in FIG. 1 by a waveform that transitions between low and high values without distortion. In the real world, a variety of factors affect and undesirably distort the waveform from the idealized representation of FIG. 1. For example, digital signals communicated over relatively long transmission lines can be severely impacted by "ringing." Ringing is defined as a damped oscillation occurring in the signal as a result of distributed inductance and capacitance and their affect on signal transitions. Thus, as shown in FIG. 2, transitions in the digital waveform from low-to-high and high-to-low produce a damped oscillation immediately following the transition. In high frequency digital waveforms, the damped oscillation can last for a substantial period of the signal.
These damped oscillations in the waveform are generally undesirable because they reduce the speed of operation of the overall circuit. That is, no assurances can be given as to what the value of the digital signal will be during this damped oscillatory period, owing to the relatively large transitions in the digital waveform. Thus, if the damped oscillations are not removed from the digital waveform, then any circuitry receiving the digital waveform must wait a preselected period of time to ensure that the damped oscillations have died out sufficiently so as not to affect the value of the digital signal observed. This waiting is, of course, undesirable in that it slows the overall operation of the circuit.
Prior art devices have attempted to reduce the damped oscillations by providing a circuit to clamp the transmission line to the desired high and low levels in response to a transition in the input wave form. These circuits, however, have suffered from at least one significant shortcoming. These clamping circuits have a leakage or quiescent current that is continually present. That is, even when ringing is not present, the clamping circuit still consumes power. Leakage current is particularly problematic in electronic circuits intended for operation from a battery supply. As would be expected, leakage current in a device operating from a battery supply will significantly shorten the useable life before the battery needs to be recharged.
Additionally, these prior art circuits tend to be slow to operate, and for high frequency digital signals, slow operation of the clamping circuit may allow the most significant portion of the damped oscillation to occur before the clamping circuitry takes effect. The slow response of the clamping circuit increases the time required to stabilize the input waveform and remove the damped oscillations, slowing the operation of the overall circuit.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.